The pilot-tone system described in ITU-R BS.450 is used to transmit stereo signals from the FM transmitter. This system applies a preemphasis (high frequency boost) to the left (L) and right (R) audio channels, before using a matrix which generates a sum signal (L+R)/2 and a difference signal (L−R)/2.
The sum signal is transmitted in base band up to 15 kHz. The difference signal is transmitted in double sideband modulation, with the 38 kHz carrier suppressed. To enable the receiver to demodulate the difference signal, a pilot tone is transmitted with a frequency of 19 kHz, which corresponds to half the carrier frequency.
The signal mixture of sum-, difference, and pilot-tone signal is referred to as a multiplex signal (MPX). The MPX signal and additional signals, if necessary (such as RDS) modulate an FM transmitter's high-frequency carrier signal in its frequency. The high-frequency broadcasting is done via an antenna.
A superheterodyne FM receiver receives the high-frequency signal via an antenna. The radio-frequency signal (RF signal) of the antenna is amplified, preselected in the frequency, and moved into an intermediate frequency (IF) range.
An intermediate frequency filter lets through most of the usable signal bandwidth and filters out most adjacent channel interference. By subsequent amplitude limiting of the intermediate frequency signal in a limiter, the amplitude fluctuations in the RF- and/or IF-signal reception are suppressed.
Subsequently, a frequency demodulation takes place that delivers the MPX signal. This is fed into a stereo decoder.
A block diagram of an MPX stereo decoder is shown in FIG. 1.
A mono receiver evaluates only the sum signal (L+R)/2 in the baseband extending up to 15 kHz. In a stereo receiver, a stereo decoder obtains the L and R signals from the MPX signal.
In the stereo decoder, a frequency doubling of the pilot tone signal takes place, and hence a recovery of the carrier frequency 38 kHz of the difference signal occurs. The stereo decoder demodulates the double sideband-modulated difference signal and thus recovers the signal (L−R)/2. The sum signal (L+R)/2 is recovered directly from the baseband. By dematrixing, meaning addition or subtraction of these two signals, the decoder recovers the preemphasized L and R signals again. These are then subjected to a deemphasis that compensates for the transmitter-side preemphasis. The original signals L and R are thus available.
Other decoding methods, such as the switching-decoder, differ from the above-depicted signal processing with regard to demodulation and dematrixing; however, they can be converted in the above model as seen in signal theory.
The receiver behavior according to current technology is as follows.
The FM pilot tone system should first be considered in theory with respect to noise.
The constant noise density in the RF- or IF-range is converted by the FM-demodulation process into a frequency-proportional voltage-density.
The MPX-spectrum and noise voltage density (Rauschspannungsdichte) are shown in FIG. 2.
It can be seen from FIG. 2 that the difference signal contains significantly more noise between 23 and 53 kHz than the sum signal, which only reaches up to 15 kHz.
The monaural audio signal-to-noise ratio SNRFM, prevailing after the FM demodulation with respect to +/−75 kHz frequency deviation without consideration of a pre/deemphasis, can be approximated by the following formula:SNRFM=3β2((β+1)CNR with the radio-frequent carrier-to-noise-ratioCNR=A2/(2BTN0)β is the FM modulation indexA is the amplitude of the carrier signalN0/2 is the two-sided spectral noise power density with white noiseBT is the radio frequency transmission bandwidthIt can be estimated using the Carson formula by way ofBT=2((β+1)W W is the audio signal bandwidthThe result obtained with the Carson formula is β+1=BT/2 WUsed in the formula for SNRFM results for β>>1 inSNRFM=3CNR(BT/2W)3 
The above-mentioned formulas apply above the so-called FM threshold, below which the signal quality decreases rapidly and impulse-noise can be expected, which results in clicks or crackling after demodulation.
The FM threshold at a radio frequency transmission bandwidth of 180 kHz is approximately 11 dB CNR. Above this threshold is:SNRFM=28 dB+CNR with mono receptionSNRFM=5 dB+CNR with stereo reception
10 dB can further be added when considering a preemphasis/deemphasis of 50 μs respectively 13 dB at 75 μs.
The FM threshold of approximately 11 dB corresponds to a mono audio-to-noise ratio of 39 dB+deemphasis-gain. In case of a deemphasis of 50 μs there is likely to be at least a 49 dB mono audio signal-to-noise ratio, or 26 dB stereo audio signal-to-noise ratio. With regard to a 40 kHz frequency deviation, an audio signal-to-noise ratio of 43.5 dB mono and 20.5 dB stereo is to be expected. The mono-gain in the audio signal-to-noise ratio at the FM threshold is 23 dB. In the receiver, the mono-gain [N(mono)−N(stereo)] decreases with increasing audio signal-to-noise ratio, as can be seen from the limiter curve of an exemplary FM receiver shown in FIG. 3.
The audio signal-to-noise ratio SNR is limited upwards by the inherent noise of the rest of the transmission chain. In FIG. 3 the solid curve N (stereo) shows the size of the noise N of an FM stereo reception. The dashed line N shows the function “stereo blend” which reduces the level of the difference signal
according to a falling of the antenna input voltage below a threshold (here about 100 μV antenna voltage). The noise power N is kept at a reduced level and does not rise further. The result is an increasing deterioration of the L-R channel separation (stereo blend) up to mono (L=R, i.e., no channel separation).
From approximately 40 μV, the useful signal reaches its full level. The distance from the curve N to the curve S+N is the audio signal-to-noise ratio.
According to current technology, a reduction of the level of the difference signal is used to raise the audio signal-to-noise ratio at the expense of the L-R channel separation. The reduction can be made broadband or in frequency ranges, such as in the high frequencies, and depends on the extent of external signals, external criteria, or an estimate of the interference signal.
Further actions in the receiver to reduce the audibility of interferences in the audio frequency range or MPX range are lowering of the higher audio frequencies (hi-blend, hi-cut) during strong noise, and volume-reduction or muting (muting, noise blanker) during strong interference. These also have an effect on the sum signal (mono signal).